1. Field of the Invention
The present invention relates to a power-supply unit of a DC-DC converter and, in particular, concerns to efficiency improvement of an isolation type DC-DC converter under a light load thereof.
2. Description of Related Art
A power-supply unit of a DC-DC converter is required to have high efficiency of power conversion in a wide load range from energy saving/energy conservation point of view. Therefore, a power-supply unit having a full bridge circuit at the primary side and a current doubler type rectifier and smoothing circuit at the secondary side is used to attain the high efficiency of power conversion with a soft-switching technology which achieves ZVS (Zero Voltage Switching, hereinafter referred to as ZVS) using a resonant inductor Lr and a resonant capacitor Cr of the full bridge circuit. However, to achieve the ZVS becomes difficult under a light load using this type of power-supply unit, and the efficiency of power conversion decreases. As a method for solving the forgoing problems, Japanese Patent Laid-Open Publication No. 2004-260928 and Japanese Patent Laid-Open Publication No. 2002-66739 have been known. Both the applications describe a method for achieving the ZVS by increasing a value of the resonant inductor under a light load.
In the Japanese Patent Laid-Open Publication No. 2004-260928, a second resonant inductor and a parallel circuit of a switch are provided in series with the resonant inductor Lr for detecting a load-state, and if the load is a light load, the switch is turned off to increase the value of the resonant inductor step by step to achieve the ZVS. However, in the method, a circuit component is added to a power block of the power-supply unit, thereby the circuit becomes complex.
In addition, in the Japanese Patent Laid-Open Publication No. 2002-66739, a saturable reactor is used for increasing the value of the resonant inductor continuously, and the value of the resonant inductor is continuously varied by linearly controlling the saturable reactor based on the load-state. In this case, a circuit for the linear control is added as well as the circuit component is added to the power block of the power-supply unit. Then, the circuit becomes more complex, and a drive loss (energy loss) due to the linear control is generated even under a light load.
It is, therefore, an object of the present invention to solve the forgoing problems, and to provide a power-supply unit of an isolation type DC-DC converter which has high efficiency of power conversion even under a light load.